1. Field of the Invention
The present invention relates to a novel piezoelectric resonator and electronic components containing such novel piezoelectric resonators, and more particularly, to a novel piezoelectric resonator which maximizes the effective use of mechanical resonance of a piezoelectric member, and electronic components containing such a novel piezoelectric resonator, such as a ladder filter, an oscillator, a discriminator, and a filter.
2. Description of the Related Art
FIG. 18 is a perspective view of a piezoelectric resonator which the applicant has disclosed in Japanese patent application No. 8-110475 filed in the Japanese Patent Office and which has not yet been published or laid open. Japanese Patent Application No. 8-110475 corresponds to U.S. patent application Ser. No. 08/829,597, (Attorney Docket No. 36856.22) for "PIEZOELECTRIC RESONATOR AND ELECTRONIC COMPONENT CONTAINING SAME," the disclosure of which is hereby incorporated by reference. FIG. 19 is a view showing the internal structure of the piezoelectric resonator shown in FIG. 18. A piezoelectric resonator 1 preferably includes a base member 2 having, for example, a substantially rectangular-parallelpiped shape. The base member 2 is formed by integrally laminating a plurality of piezoelectric layers with electrodes 3 disposed therebetween. The piezoelectric layers are preferably made from piezoelectric ceramic. The electrodes 3 are disposed such that major surfaces thereof are substantially perpendicular to a longitudinal direction of the base member 2 and such that the electrodes 3 are spaced by a certain interval therebetween.
The piezoelectric layers are polarized in opposite directions at both sides of electrodes 3 in the longitudinal direction of the base member 2 as shown in FIG. 19, and thereby, a vibrating section 4 is formed as shown by hatching in FIG. 19. Since electrodes are not disposed at the opposite ends of the base member 2 in the longitudinal direction in the piezoelectric resonator 1, the piezoelectric layers disposed at the opposite ends of the base member 2 serve as piezoelectrically inactive dummy layers "d."
On opposite side surfaces of the base member 2, a plurality of insulating films 5 and 6 are disposed, respectively. On one side surface of the base member 2, alternate exposed portions of the electrodes 3 are covered by the insulating film 5. On the other side surface of the base member 2, alternate exposed portions of the electrodes 3 which are not covered by the insulating film 5 on the above-described side surface are covered by the insulating film 6.
On the side surfaces of the base member 2 on which the insulating films 5 and 6 are disposed defining connection sections, external electrodes 7 and 8 are provided. Therefore, the external electrode 7 is connected to electrodes 3 which are not covered by the insulating film 5 and the external electrode 8 is connected to electrodes 3 which are not covered by the insulating film 6. In other words, adjacent electrodes among the electrodes 3 are connected to the external electrodes 7 and 8, respectively. The external electrodes 7 and 8 are used as input and output electrodes.
FIGS. 20A and 20B illustrate vibration conditions in the piezoelectric resonator shown in FIGS. 18 and 19. The piezoelectric resonator 1 shown in FIGS. 20A and 20B has the same structure as the piezoelectric resonator described above.
Among three rows of arrows shown in the base member 2 of the piezoelectric resonator 1 in FIGS. 20A and 20B, the upper row of arrows indicate the direction in which an electric field is applied, the middle row of white arrows indicate the direction of polarization, and the lower row of arrows indicate the direction in which each piezoelectric layer expands and contracts in the stiffened piezoelectric resonator.
when an AC signal, which changes its direction of voltage as time elapses, is applied to the piezoelectric resonator 1, the base member 2 alternately changes its condition between the conditions shown in FIG. 20A and FIG. 20B to vibrate in the longitudinal direction. In other words, when an electric field is applied to each piezoelectric layer in the same direction as the direction of polarization as shown in FIG. 20A, each piezoelectric layer expands in the longitudinal direction of the base member 2 and the base member 2 as a whole expands in the longitudinal direction. On the other hand, when an electric field is applied to each piezoelectric layer in the direction opposite to the direction of polarization as shown in FIG. 20B, each piezoelectric layer contracts in the longitudinal direction of the base member 2 and the base member 2 contracts as a whole in the longitudinal direction. These operations are repeated, and the base member 2 vibrates in the longitudinal direction.
The piezoelectric resonator 1 has a structure in which each piezoelectric layer expands and contracts in the same direction when an electric field is applied, as shown in FIGS. 20A and 20B. In other words, each piezoelectric layer has a driving force in the same direction, generated due to the inverse piezoelectric effect. Therefore, an input signal is efficiently converted to mechanical vibration in the piezoelectric resonator 1, and it can have a larger electromagnetic coupling coefficient and also have a relatively large .DELTA.F, which is a difference between the resonant frequency Fr and the antiresonant frequency Fa. A resonator having a large .DELTA.F is well suited for use, for example, as a wide-frequency-band filter.
A narrow-frequency-band filter and an oscillator may, however, require a small .DELTA.F. To obtain a small .DELTA.F in the piezoelectric resonator 1 described in FIGS. 18 to 20B, the area of each electrode, or the size or thickness of the piezoelectric layers must be changed which is difficult and requires a significant amount of time and effort.